Lapping is a well-established process for finishing the tooth surfaces of certain types of gear sets. It is particularly well suited for gear sets comprising a ring gear and associated pinion gear, such as such as bevel or hypoid gear sets. Such gear sets are frequently employed in automotive applications, such as driveline components, including the ring and pinion gear associated with a differential in an automotive vehicle
In the lapping process, a ring gear and pinion gear are each typically mounted to rotatable spindles. These spindles are also frequently adapted such that either the ring gear, pinion gear or both may be translated back and forth with respect to the other gear (i.e., via pinion-cone motion, gear-cone motion and vertical and horizontal mounting offsets). While the gears of the gear set are in meshing engagement, an abrasive lapping compound is introduced to the gear such that the rotation, location, speed and torque of the gears causes the abrasive in the lapping compound to remove or lap material from the faces of the gear teeth. The purpose of lapping is to improve the performance of the gear set in application with regard to criteria which are frequently referred to as noise, vibration and harshness (NVH). NVH is an automotive industry term associated with the treatment of vibration and audible sounds. Harshness usually refers to treatments of transient frequencies or shock.
The manufacturing processes employed to make the ring gear and pinion gear are complex processes comprising numerous metal-working, heat treatment and finishing processes. Following the manufacturing process, either the ring gear, pinion gear, or both, may not conform to their design specifications. Such non-conformance means that the teeth of these gears may not properly mesh with one another. This non-conformance also produces vibration energy as the teeth of the gears are rotated while in mesh. This vibration energy can produce various vibration waveforms, including audible noise. One type of waveform resembles a series of vibration energy spikes or pulses and can be caused by discrete defects, such as nicks (positive material or dents in the gear flank that cause a raised material condition on the surface) or other asperities on the gear face. If a nick is present where the two mating gears contact, the resulting disturbance may be heard or felt by a vehicle operator depending on the insulation of the underbody and the severity of nick or defect. Another type of waveform is more continuous in nature and results from mesh of the gears as they are rotated, and is frequently referred to as the mesh frequency. The NVH performance of the vehicle is strongly influenced by the amount of vibration energy generated when the pinion and ring gear are rolled together in meshing engagement. Therefore, a variety of techniques are currently employed, such as grinding and lapping, to remove defects such as nicks, eliminate index and pitch variations, and improve the degree to which the teeth of the ring gear and pinion gear mesh smoothly and continuously with one another, thereby improving the NVH performance of vehicles into which these components are installed.
In order to assess the NVH performance of gear sets, various testing machines and methods are employed at various stages of manufacture and assembly. For example, various nick detection approaches are currently offered by gear equipment vendors. One nick detection approach utilizes the linear displacement of a glass scale or the radial displacement of a photo-encoder on the central and/or radial axis of the ring gear during a pre-roll of a lapping operation (i.e. pre-roll occurs before lapping and during run out inspection on a CNC machine). Another vendor utilizes a similar concept of detection, and both correlate the data to a spike seen in the run out curve and accumulated pitch. Both vendors establish their threshold limits that define a nick around these generated curves. However, this inspection technique adds time to the current manufacturing cycle, and utilizes a low speed (roughly 100 rpm) double flank roll at the mounting distance of the gear set. In addition, the sensitivity of the measurement is such that it does not detect all nicks that are capable of diminishing the NVH performance of a gear set. As a further example, in order to address NVH concerns in the gearing stage, gear sets are sometimes rolled together using a basic perpendicular spindle machine and the gear sets are then examined by a trained operator for acceptable contact pattern position and operating noise levels. This analysis occurs after the final machining operation of a gear and/or gear set—lapping or grinding. Any remaining NVH disturbance due to nicks or the overall mesh must be located visually and audibly by the operator, and then must be removed physically by a hand grinder. In some cases, due to teeth cutting errors, heat treatment distortion and other causes, the gear set must be scrapped.
As another example, hypoid gear sets are commonly tested using a single flank test, which is a process that inspects transmission error and rotational characteristics in the dynamic condition. The transmission error is measured based upon the premise that the transmitted dynamics from the pinion to the ring gear will have some deviation from the theoretical transmission. Photo encoders and linear glass scales on these test machines inspect this deviation and record it in terms of arc seconds of radial displacement. These testing machines may also employ vibration sensors which are adapted to produce an output signal in response to sensed vibration energy that is produced while the gears are rotated while in mesh.
The vibration sensor or transducer is frequently a piezo-electric accelerometer and produces an output signal that is proportional to the magnitude of the vibration energy produced by the gear set rotating in mesh. This output signal may be used in several ways. One way is to observe the amplitude of the characteristic time-based waveform of the vibration sensor. Another way of using the vibration sensor is to perform a Fast Fourier Transform (FFT) of the output signal or to observe the characteristic response of the gear set during rotation in the frequency domain, such as over a range of rotation frequencies and other characteristic variables associated with the meshed gears, including the torque applied to the pinion or the ring gear, any braking torques applied to either the ring gear or the pinion gear, backlash, the degree to which the gears are meshed with one another and other factors.
While the typical single flank testing machines incorporating these vibration sensors provide useful information, they have a number of limitations. First, in order to test gears in conjunction with the use of lapping to improve their NVH characteristics, it is necessary to remove the gear sets from the lapping machines, remove the lapping compound, set them up in mesh on a testing machine, and then run various tests to determine the NVH characteristics of the gear set being lapped. This lapping/testing approach is undesirable in a high volume production environment because of the cost and time associated with transitioning the gear sets from the production lapping equipment to the testing equipment and back again if need be for additional lapping and/or testing. In addition, most single flank testing is done at rotational speeds that are significantly lower than the rotational speeds at which lapping is done and at which the gear sets will be used in their final application. While attempts have been made to correlate the amplitude of the time-based waveform and/or the frequency-based FFT output produced by testing with the NVH characteristics of the gear sets in the lapping environment or in their final applications, such efforts have had very limited success, because of the complexity of the factors to be considered, including the variability in the set-up of the gear set in manufacturing, final assembly and application, the broader range of rotational speeds in manufacturing and application versus testing, contact over a range of positions on the gear flank in manufacturing and application versus just one position during testing, as well as other factors.
As a final example, in areas of final assembly, NVH analysis and acceptance are being utilized on both the carrier assembly and the final axle assembly utilizing vibration sensors to sense and record any vibrations. However, all of these test methods have the disadvantage of being separate from the lapping process, and thus add cost and time to the production cycle. Also, they provide relatively little quantitative information about the defects detected, the necessary corrective action required and whether a particular corrective action has produced the desired effect, absent retesting once the corrective measure has been taken.
In addition to the limitations of existing test equipment and methods, customers are continually placing a higher demand on NVH performance of gear sets and making the related acceptance criteria more stringent. As such, it is highly desirable to establish a lapping apparatus and lapping methods that provide an accurate indication of the vibration energy characteristics of the gear set while it is in the production lapping equipment and during the lapping process so that the production of gear sets that will have acceptable NVH characteristics in their final application can be assured. It is also highly desirable to provide feedback control of the lapping process by measurement of the characteristic vibration energy output of the gear set while it is being lapped.
It is therefore desirable to realize an apparatus and method for lapping that may be controlled based on the vibration energy output of the gear set while it is lapping. It is also desirable to provide an apparatus and method that provides closed loop control of the lapping process based on the vibration energy output from the gear set. It is also desirable to identify gears sets that may not be corrected by lapping alone, during the lapping process, so as to avoid the manufacturing, assembly and other costs associated therewith.